The present invention relates to an apparatus as defined in the preamble of claim 1, and a method as defined in the preamble of claim 5, i.e. a method and an apparatus for charging a piezoelectric element.
The present piezoelectric elements being considered in more detail are, in particular but not exclusively, piezoelectric elements used as actuators. Piezoelectric elements can be used for such purposes because, as is known, they possess the property of contracting or expanding as a function of a voltage applied thereto or occurring therein.
The practical implementation of actuators using piezoelectric elements proves to be advantageous in particular if the actuator in question must perform rapid and/or frequent movements.
The use of piezoelectric elements as actuators proves to be advantageous, inter alia, in fuel injection nozzles for internal combustion engines. Reference is made, for example, to EP 0 371 469 B1 and to EP 0 379 182 B1 regarding the usability of piezoelectric elements in fuel injection nozzles.
Piezoelectric elements are capacitative elements which, as already partially alluded to above, contract and expand in accordance with the particular charge state or the voltage occurring therein or applied thereto. In the example of a fuel injection nozzle, expansion and contraction of piezoelectric elements is used to control valves that manipulate the linear strokes of injection needles. The use of piezoelectric elements with double acting, double seat valves to control corresponding injection needles in a fuel injection system is shown in German patent applications DE 197 42 073 A1 and DE 197 29 844 A1, which are incorporated by reference herein in their entirety.
Fuel injection systems using piezoelectric actuators are characterized by the fact that, to a first approximation, piezoelectric actuators exhibit a proportional relationship between applied voltage and the linear expansion. In a fuel injection nozzle, for example, implemented as a double acting, double seat valve to control the linear stroke of a needle for fuel injection into a cylinder of an internal combustion engine, the amount of fuel injected into a corresponding cylinder is a function of the time the valve is open, and in the case of the use of a piezoelectric element, the activation voltage applied to the piezoelectric element.
FIG. 8 is a schematic representation of a fuel injection system using a piezoelectric element 2010 as an actuator. Referring to FIG. 8, the piezoelectric element 2010 is electrically energized to expand and contract in response to a given activation voltage. The piezoelectric element 2010 is coupled to a piston 2015. In the expanded state, the piezoelectric element 2010 causes the piston 2015 to protrude into a hydraulic adapter 2020 which contains a hydraulic fluid, for example fuel. As a result of the piezoelectric element""s expansion, a double acting control valve 2025 is hydraulically pushed away from hydraulic adapter 2020 and the valve plug 2035 is extended away from a first closed position 2040. The combination of double acting control valve 2025 and hollow bore 2050 is often referred to as double acting, double seat valve for the reason that when piezoelectric element 2010 is in an unexcited state, the double acting control valve 2025 rests in its first closed position 2040. On the other hand, when the piezoelectric element 2010 is fully extended, it rests in its second closed position 2030. The later position of valve plug 2035 is schematically represented with ghost lines in FIG. 8.
The fuel injection system comprises an injection needle 2070 allowing for injection of fuel from a pressurized fuel supply line 2060 into the cylinder (not shown). When the piezoelectric element 2010 is unexcited or when it is fully extended, the double acting control valve 2025 rests respectively in its first closed position 2040 or in its second closed position 2030. In either case, the hydraulic rail pressure maintains injection needle 2070 at a closed position. Thus, the fuel mixture does not enter into the cylinder (not shown). Conversely, when the piezoelectric element 2010 is excited such that double acting control valve 2025 is in the so-called mid-position with respect to the hollow bore 2050, then there is a pressure drop in the pressurized fuel supply line 2060. This pressure drop results in a pressure differential in the pressurized fuel supply line 2060 between the top and the bottom of the injection needle 2070 so that the injection needle 2070 is lifted allowing for fuel injection into the cylinder (not shown).
In a fuel injection system it is the goal to achieve a desired fuel injection volume with high accuracy, especially at small injection volumes, for example during pre-injection. In the example of a double seat valve, the piezoelectric element is to be expanded or contracted by the effect of an activation voltage so that a controlled valve plug is positioned midway between the two seats of the double seat valve to position the corresponding injection needle for maximum fuel flow during a set time period. It has proven to be difficult to determine and apply an activation voltage with sufficient precision such that the corresponding valve plug is accurately positioned for maximum fuel flow.
It is therefore an object of the present invention to develop the apparatus as defined in the preamble of claim 1 and the method as defined in the preamble of claim 5 in such a way that an activation voltage level for a piezoelectric element is determined and set with sufficient precision to accurately position a valve plug for maximum fuel flow. The piezoelectric element can be one of several piezoelectric elements used as actuators in a system such as, for example, a fuel injection system.
This object is achieved, according to the present invention, by way of the features claimed in the characterizing portion of claim 1 (apparatus) and in the characterizing portion of claim 5 (method).
These provide for:
an activation voltage value for charging the piezoelectric element to be set as a function of a measured operating characteristic of the fuel injection system (characterizing portion of claim 1); and for
a definition to be made, prior to charging, as to a value for an activation voltage for charging the piezoelectric element, as a function of a measured operating characteristic of the fuel injection system (characterizing portion of claim 5).
The amount of force needed to move the valve needle is a function of the operating characteristics of the fuel injection system, for example, the fuel pressure applied to the control valve at the fuel injection nozzle, temperature, and so on. Thus, the load on the piezoelectric element from the corresponding valve, and the amount of displacement of the actuator in response to application of a particular activation voltage are also a function of, for example, the fuel pressure applied to the valve.
In the case of a common rail fuel injection system, the fuel pressure at any particular fuel injection for a cylinder will be approximately equal to the fuel pressure in the common rail. The common rail fuel pressure acting upon the valves of an internal combustion engine can change significantly as a function of the working point within the fuel injection system, resulting in considerable changes in the forces acting upon the valve.
Accordingly, in this example, the activation voltage level for a piezoelectric element, suitable for displacement of the element sufficient to move the injection needle to an optimum midway position for maximum fuel flow, in the example of a double acting valve, is influenced by fuel pressure levels and changes in the level.
Given an activation voltage level set as a function of an operating characteristic of the fuel injection system such as, for example, fuel pressure, the control valve can be controlled with sufficient accuracy independently of the rail pressure, and therefore of the operating state of the system. The activation voltage applied to a piezoelectric element at any particular time will be appropriate relative to the rail pressure at the time of activation, so that the injection needle is properly positioned by the control valve for maximum injection volume. In this manner, a desired injection volume can be achieved with sufficient accuracy even if the injection volume is small or the injection profile complex.
Advantageous developments of the present invention are evident from the dependent claims, the description below, and the Figures.